166. Fraction to Recurring Decimal

Posted 2020-10-09 为了更优秀的你，加油！

Given two integers representing the numerator and denominator of a fraction, return the fraction in string format.

If the fractional part is repeating, enclose the repeating part in parentheses.

For example,

• Given numerator = 1, denominator = 2, return "0.5".
• Given numerator = 2, denominator = 1, return "2".
• Given numerator = 2, denominator = 3, return "0.(6)".

解题思路：一个循环小数，必然是由于余数循环开始的。整数部分肯定是有限的，循环是从小数部分或者说余数部分开始的，于是设立个map，每次把余数映射到当前res的最末尾，如果开始循环之后，在此位置放左括号。

class Solution {
public:
    string fractionToDecimal(int numerator, int denominator) {
        if(numerator==0)return "0";
        string res;
        if((numerator>0)^(denominator>0))res+=‘-‘;
        long long int n=abs((long)numerator),d=abs((long)denominator),r=n%d;
        res+=to_string(n/d);
        if(!r)return res;
        res+=‘.‘;
        unordered_map<long long, long long>hash;
        for(;r;r=r%d){
            if(hash[r]){
                res.insert(hash[r],1,‘(‘);
                res+=‘)‘;
                break;
            }
            hash[r]=res.size();
            r*=10;
            res+=to_string(r/d);
        }
        return res;
    }
};